Non-aqueous electrolyte batteries including an organic solvent such as lithium-ion batteries have a higher energy density and a wider operating temperature range, compared to batteries including an aqueous solution such as alkaline batteries and nickel-metal hydride secondary batteries. In recent years, due to rapid progression in technology in the electronics field, miniaturization of electronic devices is increasing. With such miniaturization, demand is increasing for small and lightweight non-aqueous electrolyte batteries having a high energy density, for use as the power source for electronic devices. Specifically, for mobile phones and laptop computers, cylindrical or prismatic lithium-ion secondary batteries are used; and for calculators and wristwatches, coin-shaped lithium batteries are used as the main power source. Furthermore, lithium-ion batteries are beginning to be used as the power source for electric vehicles (EV) and hybrid electric vehicles (HEV). With the above in high demand, high and long-term reliability is expected of non-aqueous electrolyte batteries.
For the sealing portion of coin-shaped as well as cylindrical non-aqueous electrolyte batteries, a gasket formed of a widely-used resin such as polypropylene or polyethylene is used. However, in contrast to batteries including an aqueous solution, non-aqueous electrolyte batteries are very sensitive to moisture, and entry of moisture from the outside causes early degradation of battery characteristics.
For example, lithium-ion secondary batteries use a positive electrode active material with a high potential of 4 V or more relative to lithium metal; and use LiPF6 or LiBF4 as the supporting salt in the electrolyte, in view of tolerance to such high potential. Such supporting salt has high reactivity with moisture; and the strong acid produced by a reaction causes degradation of battery performance. In the case of batteries including lithium metal, a reaction between lithium and moisture causes loss of lithium activity and degradation of battery performance.
Thus, there has been a study to use a copolymer (PFA resin) of tetrafluoroethylene and perfluoroalkyl vinyl ether with low moisture vapor permeability as the gasket, to prevent entry of moisture from the outside into the battery (see Patent Literature 1). There has also been a study to determine the fluorine content in the PFA resin and the melt flow rate range of the PFA resin, to improve battery characteristics in a high humidity environment as well as improving the leak-proof characteristic (see Patent Literature 2).